Coughing, convulsing and calls for probe after Arkansas execution

April 28 (Reuters) - A lawyer for an executed Arkansas death row inmate asked the state on Friday to investigate why his client coughed and convulsed on a death chamber gurney, saying a lethal injection drug may have been the cause.

Kenneth Williams was the fourth inmate put to death in eight days in the state, which before April had not carried out an execution in 12 years. Accounts of his execution on Thursday night raised fresh concerns about whether the sedative midazolam, a Valium-like drug, is effective in lethal injection mixes.

Witnesses said Williams, who admitted to killing four people, jerked and gasped for air for about 30 seconds a few minutes after his execution began. The state said it was a routine execution lasting about 15 minutes, but critics said something was amiss.

"It is not a normal reaction to therapeutic doses of midazolam," said Jonathan Groner, a professor of surgery at the Ohio State University College of Medicine who has testified against the drug's use in executions.

"Was the drug doing what the state intended it to do or was the person being chemically waterboarded on the way to being killed?" he asked in an interview when talking about execution mixes.

Shawn Nolan, a lawyer for Williams, and the American Civil Liberties Union on Friday asked Arkansas to investigate.

"This is very disturbing, but not at all surprising, given the history of the risky sedative midazolam, which has been used in many botched executions," Nolan said in a statement.

Republican Arkansas Governor Asa Hutchinson, who set the hurried execution schedule because the state's supply of midazolam expires at the end of April, told reporters on Friday there was no need for an investigation and all the executions this month were carried out within the state's protocols.

The United Nations' human rights office voiced deep concern on Friday, saying the state's rush to carry out the executions before a drug expired added to the "arbitrariness and cruelty" of the process.

Midazolam is supposed to render an inmate unconscious. But critics contend it has failed to render inmates insensate in some cases, leaving them to feel the effects of the two other drugs in the execution mix, a paralytic that halts breathing and another drug that stops the heart while causing an excruciating burning sensation.

Death penalty states once used an anesthetic in their mixes until major pharmaceutical companies began a sales ban about six years ago to prison systems due to ethical concerns.

In response, several states turned to new mixes that included midazolam. The drug was used in troubled executions in Oklahoma and Arizona where witnesses said inmates twisted in pain on death chamber gurneys.

Death penalty supporters have said some pain in executions is warranted given the brutality of the murders the condemned typically commit and the harm they have inflicted on victim's families.

In 2015, the U.S. Supreme Court upheld the use of midazolam in a case brought by Oklahoma death row inmates, saying the inmates failed to show there was an available alternative method of execution that would be less painful. (Reporting by Jon Herskovitz; Additional reporting by Steve Barnes in Grady, Arkansas and Stephanie Nebehay in Geneva; Editing by Colleen Jenkins and Tom Brown)